Method of producing electrographic image from original provided with a conductivity pattern

ABSTRACT

The invention provides an electrographic image by generating between a first and second electrode an electric field across a developer powder sandwiched between an original having a conductivity pattern of maximum and minimum conductivities and an image carrier of uniform conductivity intermediate the maximum and minimum conductivities of said pattern.

United States Patent [191 Eantarano 1March 20, 1973 METHOD OF PRODUCING ELECTROGRAPHIC IMAGE FROM ORIGINAL PROVIDED WITH A CONDUCTIVITY PATTERN Inventor:

[76] Costantino Marcus Cantarano, 49,

avenue Franklin Roosevelt, Thiais, France Filed: Dec. 8, 1969 App1.No.: 870,404

Related US. Application Data Continuation-impart of Ser. No. 631,792, April 8, 1967, abandoned.

References Cited UNlTED STATES PATENTS Allinger et al ..117/17.5 MongrietT-Yeates ..1 17/17.5

3,427,242 2/ l 969 Mihajlov ..96/1.3 2,924,519 2/ 1 960 Bertelsen.... ..96/1.4 3,284,224 11/1966 Lehmann ...1 17/37 LE 3,615,383 10/1971 Inoue et a1 ..117/37 LE 3,234,904 2/1966 Van Wagner ..118/638 3,166,418 1/1965 Gurdlach ..'.117/17.5 2,951,443 9/1960 Byme l7/l7.5 2,968,553 1 1961 Gundlach ..96/ 1.4 3,093,039 6/1963 Rheinfrank ..l 17/1 7.5 3,132,963 5/1964 Jarvis ..117/17.5 3,147,147 9/1964 Carlson ..117/17.5 3,247,794 4/1966 Zabiak ..117/17.5 3,326,709 6/1967 Nail ......117/17.5 3,368,894 2/1968 Matkan et al. ..96/1.4 3,013,890 12/1961 Bixby ..L ..l17/17.5 3,132,037 5/1964 Hunter ..117/17.5 3,328,193 6/1967 Oliphant et a1 ..117/37 LE Primary ExaminerWilliam D. Martin Assistant Examiner-M. Sofocleous Attorney-John W. Malley et a1.

57 ABSTRACT 10 Claims, 11 Drawing Figures PMENTEBmzmm 3,721,551

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* SHEET 3 BF 3 IMP/liar CbAIS AA/T MO A Kat/s QWAM' METHOD OF PRODUCING ELECTROGRAPIIIC IMAGE FROM ORIGINAL PROVIDED WITH A CONDUCTIVITY PATTERN This application is a continuation-in-part of my copending application Ser. No. 631,792, filed Apr. 8,1967 and now abandoned.

This invention relates to the production of electrographic images from an original providedwith a conductivity pattern and to the transfer of the obtained images on to sheets or webs of copy material.

As used herein, the term conductivity pattern is to be understood as including any virtually plane surface formed by parts having different electric conductivities.

In the following specification, the term insulating is to be understood as defining the quality of having an electric conductivity lower than mho/cm and the term non-insulating as defining the quality of having an electric conductivity superior to 10 mho/cm.

In the actual art, a feature of electrographic methods resides in the use of an original provided with a conductivity pattern including high insulating parts which will selectively hold electric charges to form a latent electrostatic image thus an electrographic image may be developed by an electrically responsive powder which adheres to the charged parts of the latent image. This electrographic image will not be obtained in a stable way because of the passage of electric charges even through the high insulating parts of the original causing the effacement of at least a part of the latent image during the step of the development. A typical original of actual electrography consists in a photoconductive insulating layer provided with a conductivity pattern resulting from an exposure to a light image such a photoconductive layer will be a high insulator in the dark in order to obtain a conductivity pattern including the non'illuminated high insulating parts serving to develop an electrographic image according to existing methods. These photoconductive insulating layers are slow in their response to successive different exposures to the light and, consequently, they may not be used to afford high speed processes to produce successive different electrographic images.

I have found, however, that stable electrographic images may be formed and simultaneoulsy developed from any original provided with a pattern of conductive and less conductive parts in the absence of a latent electrostatic image to this end a thin layer of developer powder is sandwiched between the pattern of the original and an image carrier having a uniform electric conductivity between the maximum and the minimum conductivities of said pattern, and an electric field is generated to charge the powder from said conductivity pattern and said image carrier because of the intermediate conductivity of said image carrier, under the influence of the electric field the charged powder is electrically attracted away from the most conductive parts of said pattern to form a first stable electrographic image on said image carrier while another part of the powder is electrically attracted towards the least conductive parts of said pattern to form a second stable electrographic image thereon. One such form of electrographic method is disclosed in my co-pending application Ser. No. 631,792, filed on Apr. 18, 1967 now abandoned. The present invention thus relates to the development of stable electrographic images through the use of a uniformly conductive image carrier and to the transfer of the obtained images on to sheets or webs of copy material.

Now in accordance with the present invention, an electrographic powder image is developed on a photoconductive layer acting as the original or the image carrier of the above mentioned method thereafter, a sheet of copy material is placed against this electrographic image the photoconductive layer is excited by a uniform exposure to a light of high intensity, and an electric field is generated to electrically charge the powder from the excited photoconductive layer whereby, under the combined action of the electric field and of the light, the charged powder image is transferred on to the sheet of copy material.

According to a first embodiment of the invention, during the step of the development the photoconductive layer is exposed to a light image and thus it acts as an original provided with a conductivity pattern.

According to a second embodiment of the invention, the photoconductive layer is uniformly exposed to the light and thus it acts as a uniformly conductive image carrier. During the development, the intensity of the light is adjusted to render the uniform conductivity of the photoconductive layer between the maximum and the minimum conductivities of the pattern of the original.

In carrying out the invention, a photoconductive insulating layer may be used, although it is preferred to use a photoconductive layer having an electric conductivity superior to 10 mho/cm during the exposure serving to the transfer of the electrographic image the dark conductivity of this layer being not critical in order to develop a satisfactory image according to the invention. Such a photoconductive layer can be called photoconductive non-insulating layer to distinguish it from the photoconductive insulating layers of actual electrography. Photoconductive materials having a virtually instantaneous response and a high sensitivity to the light, as, for example, metallic selenium, cadmium sulfide, cadmium selenide and other materials of actual photoconductive cells, may advantageously constitute the photoconductive non-insulating layers used according to the invention.

An object of this invention is to improve electrographic methods and to provide means and devices for use in electrography.

Another object of this invention is to provide methods and means for the advantageous use of photoconductive non-insulating layers in electrography.

Other objects of this invention will be apparent from the following description and accompanying drawings taken in connection with the appended claims.

In the drawings FIG. I is a sectional view showing a development device comprising an original and an image carrier between two electrodes;

FIG. 2 is a schematic representation showing two electrographic images developed in the device of FIG. 1

FIG. 3 is another schematic representation showing two electrographic images developed in the device of FIG. 1

FIG. 4 is a schematic representation showing two grains of developer powder between the original and the image carrier of the device of FIG. 1

FIG. 5 is another schematic representation showing two grains of developer powder between the original and the image carrier of the device of FIG. 1

FIG. 6 is a sectional view showing a development device comprising a photoconductive layer excited by a light image, and an image carrier FIG. 7 is a schematic representation showing two electrographic images developed in the device of FIG.

FIG. 8 is a schematic representation showing two grains of developer powder between the excited photoconductive layer and the image carrier of the device of FIG. 6

FIG. 9 is a sectional view showing a development device comprising a first and a second photoconductive layer between two electrodes;

FIG. 10 is a schematic representation showing two electrographic images developed in the device of FIG.

FIG. 11 is a sectional view showing a transfer device comprising a uniformly excited photoconductive layer and a sheet of copy material,between two electrodes.

In the arrangement shown in FIGS. 1 to 5, for producing electrographic images an original 1 provided with indicia 2 having another electric conductivity than the surface 3 of the backing material 11 is disposed between two electrodes 6 and 7. Owing to the differences of electric conductivity between the materials of the parts 1 1 and 2 of original 1, the latter is provided with a conductivity pattern formed by the areas 2 of the indicia and the blank surface 3 of the backing 11. The indicia 2 may be of different types as typewriting, China ink or pencil traces, for example. Furthermore, if continuous tone electrographic images are to be produced, an original 1 will be used which is provided with different conductive indicia 2 forming dense areas and half-shadow areas, as like as a photographic picture. On the other hand, as FIG. 6 shows, an original 1 may be used which consists in a photoconductive layer 24 provided with a conductivity pattern 2, 3 resulting from an exposure to a light image the pattern 2, 3 is then formed by the illuminated conductive parts 2 and the low illuminated low conductive parts 3 of the layer 24.

In the preferred form of the invention, a layer 24 of a photoconductive non-insulating material is used which has a high sensitivity and a virtually instantaneous response to the light. Alternatively, many photoconductive non-insulating materials may be used such as, for example, metallic selenium, thallium sulfide, cadmium sulfide, cadmium selenide, lead sulfide. The sensitivity to the light of the layers of non-insulating materials is generally from the 200 microamp/lumen of layers of metallic selenium to the 1000 milliamp/lumen of cadmium selenide layers. The spectral response of metallic selenium is from the ultra-violet to the red part of the spectrum with maximum sensitivity in the ultraviolet, cadmium sulfide has virtually the same spectral response as the human eye with maximum sensitivity between yellow and green, lead sulfide has maximum sensitivity in the infra-red and 2 to 3.5 microns of wave length. Alternatively and for example, lead sulfide, lead telluride or lead telenide layers may be used according to the invention to photograph objects emitting invisible light from 2 to 20 microns of wave lenght. The use of layers having maximum sensitivity in the visible part of the spectrum is useful to reduce the losses in the transmission of light across the lens, mirrors etc. of the optical devices serving to form the light image to be reproduced. Moreover, cadmium selenide is well adapted for the high speed production of copies from successive different light images, the responses of this material to the light and to the dark being shorter than 15 milliseconds.

The above mentioned photoconductive layers may be produced, for example, by evaporating under vacuum the photoconductive material to deposit a thin uniform layer on the surface of a transparent backing. Alternatively and for example, a plate of insulating or conductive glass from 1 to 5 mm of uniform thickness or a sheet of MYLAR (registered trade mark) having a uniform thickness from 25 to 250 microns has been found useful to constitute the backing material of the photoconductive layer. The receiving surface of the backing may be rendered rough to improve the adherence of the photoconductive layer. Cadmium sulfide and cadmium selenide are evaporated under vacuum to form a layer having a uniform thickness from 15 to 25 microns. Suitable cadmium sulfide layers are sold under trade marks CDSX7 and CDSI-I35 and cadmium selenide layers under trade marks CDSEX7 and CDSEH35 by Acova Co, Paris. Furthermore, in order to produce a metallic selenium layer affixed to an insulating flexible sheet amorphous selenium is evaporated under vacuum to form a layer having a thickness of about 30 microns on the polished surface of a glass plate, thereafter a sheet of MYLAR is affixed by a transparent glue to the selenium layer, and then, while a pressure from 2 to 50 Kgr/cm is exerted between the sheet and the glass plate, the selenium layer is heated at a temperature from 200 to 2 l 6C for a period from a few minutes to one hour to develop the crystalline structure of the metallic variety of selenium the MYLAR sheet and its metallic selenium layer are then easily detached from the polished surface of the glass plate. On the other hand, when less than 4 electrographic images per second are to be produced from a high contrastful light image, as usual in the art, a photoconductive insulating layer affixed to a backing electrode may be used, in spite of the low sensitivity of this type of photoconductive layer; although, according to the invention, a photoconductive insulating layer may be used which is constituted by a thin metallic layer of about 5 microns of gold or tellurium affixed to an insulating backing and by a photoconductive layer of 50 microns of amorphous selenium evaporated on said thin metallic layer.

As FIGS. 1 and 6 show, a thin uniform layer of developer powder 5 is placed against the conductivity pattern 2, 3 of original 1 and against an image carrier 4. If the grain size of powder 5 is from 1 to 20 microns, the thickness of the layer of powder 5 will be about 50 microns, for example. Alternatively, the developer powder 5 may coat the pattern 2, 3 or the surface 14 of image carrier 4 for the uniform application of powder 5, classic spraying or cascading devices may be used, provided that a thin uniform layer of powder 5 is formed rather than a particular amount of grains. In carrying out the present invention it is expedient to use a developer powder 5 having an electric conductivity between the maximum and the minimum conductivities of the parts 2, 3 of original 1, although the exact con ductivity of the powder is not critical in order to produce satisfactory electrographic images. Alternatively, metallic or semi-conductive or thermoplastic powders have been found useful. By way of example, charcoal, stannous oxide, lead sulfide, cadmium selenide as well as other colored materials may be powdered to constitute suitable developers. For instance, a suitable developer powder can be produced by oxidizing at a temperature of about 700C a commercial bronze to obtain a black powder containing copper dioxide and other metallic oxides this powder is then passed through sieves to reduce the grain size between 2 and microns, for example, the grams of powder may be coated with stearic acid or zinc stearate, for example which will render the powder somewhat adhesive and give to its grains a thin insulating coat which prevents electric discharges between contiguous parts of the layer of powder 5 during the application of the electric field developing the electrographic images furthermore, after the development, the coated grains of powder will conserve intense residual charges improving the adherence of the electrographic images to the image carrier 4 and to the original 1, respectively. Alternatively, any other material having similar slight adhesive or insulating characters may be used to coat the grains of powder 5. The electric conductivity of copper dioxide powders is generally between about 10- and 10- rnho/cm. It is moreover possible to use commercial bronze colored powders having a conductivity from 10 to 10' rnho/cm. Other types of developer powders, such as thermoplastic powders may be used by way of example, a conductive black thermoplastic powder may be obtained from a solid polystyrene resin to this end, for example, two parts of the resin are melted at a temperature of about 160C to be intimately mixed with one part of pure carbon, and, thereafter, the mixture is cooled and powdered to obtain a developer having grains from 1 to 10 microns, for example; the conductivity of these thermoplastic powders may be varied by changing the carbon ratio in the mixture. Furthermore, other thermoplastic powders may be used which are rendered conductive, for example, by evaporating a metal to form a conductive coat on their grains.

According to the invention, the surface M of the image carrier 4 has a uniform conductivity between the maximum and the miniumm conductivities of the pattern 2, 3 of original I. For example, a sheet of conductive paper may be used as image carrier 4. Moreover, the image carrier 4 may consist in a very thin metallic layer 34 (FIG. ll) affixed to a backing material 44 there may be used, for example, a layer 34 having a uniform thickness from a fraction of a micron to a few microns of gold, silver, aluminum or tellurium evaporated under vacuum on a sheet 44 of MYLAR, for example. Alternatively, other conductive materials may be used to form the layer 34 as well as other rigid or flexible backing materials 44 may be used instead of MYLAR. Between the image carrier 4 and the electrode 6, an insulating layer may be inserted to avoid a useless consumption of heating electric current across electrodes 6 and 7 when an electric voltage is applied to terminals 9. Moreover, an insulating layer may be interposed between original I. and electrode 7.

Referring now to the arrangements shown in FIGS. ll, 2 and 4, an original I is used which is provided with conductive indicia 2 and a low conductive backing material ll due to the relative conductivities of the parts 2, 3 and 4, the contact conductance between the grain 12 (FIG. 4) of the powder 5 and the indicia 2 is higher than the contact conductance between the grain 712 and the surface 114 of image carrier 4. The contact conductance between the grain l3 and surface 14 is higher than the contact conductance between grain l3 and blank surface 3. Under the influence of an electric field generated between electrodes 6 and 7, each grain of powder 5 is electrically charged under the sign of that surface to which the contact conductance is the more, and thus it will be electrically attracted away from this surface. For this reason, irrespectively of the direction of the electric field, the powder 12 will electrically migrate from the conductive indicia 2 towards the image carrier 4,.while the powder 13 migrates from the image carrier 4 towards the low conductive blank surface 3, as shown by the arrows in FIG. 4. When, subsequently, the electrodes 6 and 7 are separated as FIG. 2 shows, to which end the electrodes 6 or 7 may be constructed as a hinged lid of a box, the powder 12 facing the conductive indicia 2 will be found to form a first electrographic image on image carrier 4 while a second electrographic image is found on the low conductive blank surface 3 of original ll. Similarly, referring to FIGS. 6 to 8, when an original I is used which consists of a photoconductive layer 24 exposed to a light image, under the influence of the electric field the powder 12 will migrate from the illuminated conductive parts 2 of layer 24 (FIG. 8) towards image carrier 4, while powder 13 migrates from image carrier 4 towards the low illuminated low conductive parts 3 of layer 24 thus, as FIG. 7 shows, the powder 12 facing the illuminated parts 2 develops a first electrographic image on the image carrier 4, while the powder 13 forms a second electrographic image on the low illuminated parts 3 of the photoconductive layer 24. Furthermore, with reference to FIGS. 1, 3 and 5, when an original 11. is used which is provided with a conductive backing II and low conductive indicia 2;, under the influence of the electric field, the powder 13 (FIG. 5) will migrate from the conductive surface 3 towards the image carrier 4, while the powder 112 migrates from the image carrier 4 towards the low conductive indicia 2 thus, as FIG. 3 shows, the powder 13 facing the blank surface 3 develops a first electrographic image on the image carrier 4, while the powder 12 forms a second electrographic image on the low conductive indicia 2.

From the foregoing explanations it becomes apparent that the formation of the electrographic images depends on the relative conductivities of the parts 2, 3 and I4 consequently, according to the proposal of the invention, the satisfactory quality of the electrographic images is irrespective of the minimum conductivity of the pattern 2, 3 and of an excess in the duration of the electric field developing the images. Thus the electrographic images are obtained in a stable way and an original 1 provided with a non-insulating conductivity pattern 2, 3 may be used in particular, a photoconductive nonlinsulating layer having a relatively high dark conductivity may be used in carrying out the invention.

Referring to FIG. 1, according to the invention, a photoconductive layer 24 may be used as image carrier 4. One of the photoconductive layers described above with reference to FIG. 6 may constitute the layer 24 of image carrier 4. As shown in FIG. 1, a transparent electrode 6 is disposed against the transparent backing 44 of layer 24. Electrode 6 consists, for example, in a thin layer of NESA a high conductive transparent varnish sold by Fittsburg Plate Glass Co, Pittsburg. The layer of NBS/t" may be supported by a transparent glass plate 16, for example. The light sources 54 uniformly illuminate the layer 24 through the electrode 6 and the backing material 44 to induce a uniform electric conductivity in the photoconductive layer 24. By means of a potentiometer 64, the intensity of the light of sources 54 is adjusted so that the uniform electric conductivity of layer 24 is between the maximum and the minimum conductivities of the pattern 2, 3 of original 1. By way of example, a CDSEX7 photoconductive layer 24 excited by a uniform illumination of 3 lux may be used as image carrier when, for example, the original 1 consists of a sheet of ordinary paper 11 having an electric conductivity of about mho/cm and carrying China ink traces. Stable electrographic images may be developed from an original 1 provided with a conductive backing 11 and low conductive indicia 2, by applying to terminals 9 a constant electric voltage from 100 to 5000 Volts, for example. By the application of this constant voltage, it is preferred to place the conductive backing ll of original 1 in electric contact with electrode 7 and to use an image carrier 4 having a non-insulating backing .44 in contact with electrode 6 this disposition of parts permits to maintain a constant electric field between the original 1 and the image carrier 4 during the application of the constant voltage. On the other hand, a variable voltage having sufficient value to ionize the air of the gap 15, between original 1 and image carrier 4, may be advantageously applied to terminals 9 when an original 1 is used which is provided with an insulating backing l 1. An impulsion of direct voltage ionizing air 15 during 0.1 to l millisecond may be applied to develop electrographic images of satisfactory quality although, the images being obtained in a stable way, a longer duration of the development will not be critical. When the images are obtained by ionizing the air 15, it will be advantageous to use, as developer 5, one of the above described powders constituted by high conductive particles provided with thin insulating coats. Moreover, whatever the backing 11 of the original will be, the satisfactory quality of continuous tone electrographic images is obtained by applying to terminals 9 two or three complete periods of an alternating or an alternatively modulated voltage producing the ionization of the air 15 For example, an alternating voltage of 50 or 60 cycles/sec is suitable although modulated voltages having frequency from 10 to 1000 cycles/sec may be applied.

Referring now to FIG. 6, according to another embodiment of this invention, a photoconductive layer 24 affixed to a transparent backing material 44 is used as original 1. Light sources 41 illuminate a document 21 to be reproduced; the light is reflected by document 21 towards objective 31 and is transmitted across a trans parent electrode 7 and backing 44 to form the optical image of document 21 on the photoconductive layer 24. The intensity of the light is adjusted so that the uniform electric conductivity of the image carrier 4 is intermediate between the maximum and the minimum conductivities of the parts 2 and 3 of layer 24 exposed to the light image. A photoconductive image carrier 4, identical to that described with reference to FIG. 1, may be used in the device of FIG. 6. The document 21 can be a sheet of paper carrying printed or typewritten matter, or a drawing, for example, although other things may be photographed such as 3,-dimensional objects, for example. Alternatively, other radiations than light may be used to form the pattern 2, 3 such as, for example, X- or gamma-rays furthermore, any other means inducing in the layer 24 a pattern of conductive parts 2 and low conductive parts 3 may be used to produce electrographic images according to invention. On the other hand, when, for example, an X-ray image 2, 3 is formed on the layer 24, a sheet of aluminium may constitute the transparent electrode 7. In order to develop stable electrographic images from an excited photoconductive non-insulating layer 24, an electric voltage ionizing the gap of air 15 between original 1 and image carrier 4 may be advantageously applied to terminals 9. Instead of this, by using a layer 24 having an overall conductivity lower than about 10' mho/cm, such as a photoconductive insulating layer 24, the ionization of air 15 is to be avoided to prevent the conductive ionized air 15 from masking the low differences in conductivity between the parts 2, 3 of the insulating layer 24 thus by using an insulating layer 24, the intensity of the electric field is maintained lower than 3 v/micron in the gap 15. Furthermore, the sensitivity to the light of layer 24 may be improved by applying a high electric potential of suitable polarity to this layer for example, the sensitivity of a layer 24 of selenium may be improved by applying 1000 volts of positive potential to layer 24 to this end, the electrode 7 may be grounded, an electronic valve 49 and a condenser 39 are used to apply said positive potential to layer 24 through the image carrier 4, and an electric transformer 19 induces an alternating signal in the secondary coil 29 to alternatively modulate the direct voltage in order to produce satisfactory electrographic images according to the invention. On the other hand, the best quality of continuous tone electrographic images is obtained by using a layer 24 which has a photoelectric linear character, such as, for example the above mentioned layer CDSH35 of cadmium sulfide the linear feature of this layer residing in the proportionality between its electric conductivity and the intensity of the exciting light. Contrastful electrographic images will be obtained when the CDSH35 layer 24 is excited, for example, by a light image rendering the parts 3 about 30 orders in magnitude less conductive than the parts 2, the minimum illumination of parts 2 being as low as 0.1 lux, for example.

Referring now to FIG. 9, electrographic images may be developed by using a second original 10, instead of the image carrier 4 of the above described methods. In the arrangement of FIG. 9, two photoconductive originals 1 and 10 are disposed between the transparent electrodes 6 and 7 the photoconductive layer 24 of original I is excited by a first light image, as well as the photoconductive layer 240 of original 10 by a second light image, although non photoconductive originals l, 10 may be used. By applying an electric voltage to terminals 9, the developer powder will electrically migrate towards original 1 if the local conductivity of layer 240 is higher than that of layer 24, it will be equally distributed between layers 24 and 240 if the conductivities of the two layers have about the same value, and it will migrate towards original if the conductivity of layer 240 is lower than that of layer 24. This embodiment of the invention may serve, for example, to modify the electrographic image produced from an original I by electrically adding supplementary signs or effacing a part of this image. In the arrangement of FIG. 9, layer 240 is excited to comprise a part 440 having said intermediate electric conductivity, the part 220 are more conductive than the conductive part 22 of layer 24, and the part 330 are less conductive than the low conductive part 33 of layer 24 as shown in FIG. 10, after the development, the layer 24 carries an electrographic image comprising the powder image 13 obtained in substantial configuration with the pattern 2, 3 of layer 24, except the sign 120 (FIG. 10) which has been 'effaced away and the sign 130 which has been added to the powder image 13. The best results are obtained by using a layer 240 having a high sensitivity to the light. By way of example, by using as layer 24 and as layer 240 two CDSX7 layers of cadmium sulfide, the maximum and the minimum illuminations of layer 24 may be 4 and L5 lux, respectively the maximum and the minimum illuminations of layer 240 about 25 and 0.1 lux, respectively the intensity of light will be about 2.5 lux on the part 440 of layer 240, in order to induce in this layer a uniform electric conductivity between the maximum and the minimum conductivities of layer 24. In this example, the part 22 of layer 24 is about 30 orders in magnitude more conductive than its part 33, the part 220 of layer 240 is about 30 orders in magnitude more conductive than the part 22 of layer 24, and the part 330 oflayer 24. The maximum conductivity of the layer 240 is about 9X10 orders in magnitude higher than its minimum conductivity in order to obtain satisfactory results.

Electrographic images may be developed by coating original II with a first colored powder and original 10 with a second differently colored powder after the development, the area 130 of original 1 and the area 1120 of original 10 will carry particules of both the differently colored powders Referring now to FIG. Ill, a sheet of copy material 8 is placed against the powder l2, I3 of an electrographic image carried by the photoconductive layer 24 affixed to a transparent backing material 44. The sheet 8 and the layer 24 are interposed between a first electrode 7 and the transparent electrode 6. The light of the sources 54 uniformly illuminate the photoconductive layer 24 to induce a uniform electric conductivity therein. An insulating layer 18 may be interposed between the sheet 8 and electrode 7 as well as a transparent second insulating layer (not shown) may be disposed between the backing material 44 and electrode 6. By generating an electric field between electrodes 6 and 7, the powder l2, 13 is charged from the uniformly illuminated layer 24 and electrically transferred on to the sheet 8. For example, the layer 24 is uniformly excited to be 50 orders in magnitude more conductive than sheet 8. Thus, for example, by using a layer 24 of amorphous selenium, a sheet 8 having an electric conductivity lower than 10' mho/cm will be used, such as, for example, a sheet of special paper coated with polyvinyl chloride. On the other hand, in accordance with the present invention, the electrographic images l2, 13 (FIGS. 2,3, 7 and 10) are developed on photoconductive non-insulating layers in order to obtain, in the device of FIG. 11, the satisfactory transfer of these images on to sheets of ordinary paper of copy; this type of paper is often constituted by a low insulating material having a conductivity from 10' to 10' mho/cm. The best quality of the image transfer may be obtained by using one of the above mentioned photoconductive non-insulating layers during the transfer, the non-insulating layer 24 may be excited to a uniform conductivity of IO mho/cm by an illumination of IO lux, for example.

According to a method of the invention, the electrographic image is first developed on the photoconductive image carrier 4 of the device illustrated with reference to FIG. 1 and, thereafter, the image carrying photoconductive layer 24 is disposed in the device of FIG. 11 to transfer the electrographic image on to the copy material 8.

According to another method of the invention, the electrographic image is first developed on the photoconductive original 1 of the device illustrated with reference to FIG. 6 and, thereafter the image carrying photoconductive layer 24 is disposed in the device of FIG. 111 to transfer the electrographic image on to the copy material 22.

According to a further method of the invention, two electrographic images are developed respectively on the originals l and of the device illustrated with reference to FIG. 9 and, thereafter, the first and the second electrographic image carrying photoconductive layers (24 and 240) are successively disposed in the device of FIG. 11 to transfer the two electrographic images on to a first and a second copy material respectively.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not intended to be limited thereby but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What I claim is:

l. A method for producing electrographic images from an original provided with a conductivity pattern formed by areas on said original having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, comprising the steps of placing a layer of developer particles capable of receiving an electric charge against said conductivity pattern and against an image carrier comprising a non-insulating photoconductive material having a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern, said layer of developer particles being sandwiched between said conductivity pattern and said image carrier, said original and said image carrier being disposed between a first and second electrode, and generating across said original and said image carrier and between said electrodes an electric field of sufficient strength to charge said particles layer from said conductivity pattern and said image carrier simultaneously whereby said layer of developer particles receives electric charges attracting a part of said particles toward said conductivity pattern to develop a first'stable electrographic image thereon and opposite electiic charges attracting the remaining particles toward said image carrier to develop a second stable electrographic image on said image carrier.

2. A method as defined in claim 1 wherein said original is formed from a photoconductive layer which is exposed to a radiation forming said conductivity pattern in said photoconductive layer.

3. A method for producing electrographic images from an original provided with a conductivity pattern formed by areas on said original having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, comprising the steps of placing a layer of developer particles capable of receiving an electric charge against said conductivity pattern and against a non-insulating photoconductive layer so that said layer of developer particles is sandwiched between said conductivity pattern and said non-insulating photoconductive layer, uniformly exposing said non-insulating photoconductive layer to radiation thereby inducing in this non-insulating photoconductive layer a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern, said original and said non-insulating photoconductive layer being disposed between a first and a second electrode and generating across said non-insulating photoconductive layer and said original and between said electrodes an electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said non-insulating photoconductive layer simultaneously whereby said layer of developer particles receives electric charges attracting a part of said particles toward said conductivity pattern to develop a first stable electrographic image thereon and opposite electric charges attracting the remaining particles toward said non-insulating photoconductive layer to develop a second stable electrographic image on said non-insulating photoconductive layer.

4. A method as defined in claim 3 wherein said original is formed from a photoconductive layer which is exposed to a radiation forming said conductivity pattern in said photoconductive layer.

5. A method for producing electrographic images from an original provided with a conductivity pattern formed by areas on said original having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, comprising the steps of placing a layer of developer particles capable of receiving an electric charge against said conductivity pattern and against a non-insulating photoconductive layer so that said layer of developer particles is sandwiched between said conductivity pattern and said non-insulating photoconductive layer, said original and said non-insulating photoconductive layer being disposed between a first and a second electrode, generating across said original and said non-insulating photoconductive layer and between said electrodes a first electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said non insulating photoconductive layer, simultaneously whereby 'an electrographic image is developed on said non-insulating photoconductive layer by a part of said particles, removing said electrographic image bearing non-insulating photoconductive layer from said first electric field, placing a copy material against the particles of said electrographic image, uniformly exposing said non-insulating photoconductive layer to radiation thereby inducing in said non-insulating photoconductive layer a high uniform conductivity and generating across said copy material and said non-insulating photoconductive layer a second electric field of sufficient strength to charge the particles of said electrographic image from said uniformly exposed non-insulating photoconductive layer thereby transferring said electrographic image onto said copy material.

6. A method as defined in claim 5 wherein, simultaneously with the generation of said first electric field, said non-insulating photoconductive layer is uniformly exposed to radiation thereby inducing in this non-insulating photoconductive layer a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern.

7. A method for producing electrographic images comprising the steps of placing a layer of developer particles capable of receiving an electric charge against a non-insulating photoconductive layer and against a conductive image carrier so that said layer of developer particles is sandwiched between said non-insulating photoconductive layer and said image carrier, exposing said non-insulating photoconductive layer to radiation thereby forming a conductivity pattern in said non-insulating photoconductive layer, said conductivity pattern being characterized by areas on said non-insulating photoconductive layer having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, said image carrier being formed from a material having a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern, said non-insulating photoconductive layer and said image carrier being disposed between a first and second electrodes, generating across said non-insulating photoconductive layer and said image carrier and between said electrodes a first electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said image carrier, simultaneously, whereby an electrographic image is developed on said non-insulating photoconductive layer by a part of said particles, removing said electrographic image bearing non-insulating photoconductive layer from said first electric field, placing a copy material against the particles of said electrographic image, uniformly exposing said non-insulating photoconductive layer to radiation thereby inducing a high uniform conductivity in said non-insulating photoconductive layer, and generating across said copy material and said non-insulating photoconductive layer a second electric field of sufficient strength to charge the particles of said electrographic image from said uniformly exposed non-insulating photoconductive layer thereby transferring said electrographic image onto said copy material.

8. A method as defined in claim 7 wherein said noninsulating photoconductive layer is exposed to an image of visible radiation thereby forming said conductivity pattern in said non-insulating photoconductive layer during the generation of said first electric field, and said first electric field is generated to form on said non-insulating photoconductive layer a positive particles image to be transferred to said copy material.

9. A method for producing electrographic images comprising the steps of placing a layer of developer particles capable of receiving an electric charge against a first and against a second non-insulating photoconductive layer so that said layer of developer particles is sandwiched between said first and said second non-insulating photoconductive layer, exposing said first noninsulating photoconductive layer to radiation thereby forming a conductivity pattern in said first non-insulating photoconductive layer, said conductivity pattern being characterized by areas on said first nominsulating photoconductive layer having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, said first and second non-insulating photoconductive layer being disposed between a first and second electrodes, generating across said first and said second noninsulating photoconductive layer and between said electrodes :1 first electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said second non-insulating photoconductive layer, simultaneously, whereby a first electrographic image is developed on said first non-insulating photoconductive layer by a part of said particles and a second electrographic image is developer on said second non-insulating photoconductive layer by the remaining particles, removing said first and second non-insulating photoconductive layers from said first electric field, placing a first copy material against the particles of said first electrographic image, uniformly exposing said first non-insulating photoconductive layer to radiation thereby inducing a high uniform conductivity in said first non-insulating photocontive layer, generating across said first copy material and said first non-insulating photoconductive layer a second electric field of sufficient strength to charge the particles of said first electrographic image from said uniformly exposed first non-insulating photoconductive layer thereby transferring said first electrographic image onto said first copy material, placing a second copy material against the particles of said second electrographic image, uniformly exposing said second non-insulating photoconductive layer to radiation thereby inducing a high uniform conductivity in said second non-insulating photoconductive layer, generating across said second copy material and said second non-insulating photoconductive layer a third electric field of sufficient strength to charge the particles of said second electrographic image from said uniformly exposed second non-insulating photoconductive layer thereby transferring said second electrograthic image onto said second copy material.

1 v A me od as defined in claim 9, wherein, simultaneously with the generation of said first electric field, said second non-insulating photoconductive layer is uniformly exposed to radiation thereby inducing in this second non-insulating photoconductive layer a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern. 

2. A method as defined in claim 1 wherein said original is formed from a photoconductive layer which is exposed to a radiation forming said conductivity pattern in said photoconductive layer.
 3. A method for producing electrographic images from an original provided with a conductivity pattern formed by areas on said original having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, comprising the steps of placing a layer of developer particles capable of receiving an electric charge against said conductivity pattern and against a non-insulating photoconductive layer so that said layer of developer particles is sandwiched between said conductivity pattern and said non-insulating photoconductive layer, uniformly exposing said non-insulating photoconductive layer to radiation thereby inducing in this non-insulating photoconductive layer a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern, said original and said non-insulating photoconductive layer being disposed between a first and a second electrode and generating across said non-insulating photoconductive layer and said original and between said electrodes an electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said non-insulating photoconductive layer simultaneously whereby said layer of developer particles receives electric charges attracting a part of said particles toward said conductivity pattern to develop a first stable electrographic image thereon and opposite electric charges attracting the remaining particles toward said non-insulating photoconductive layer to develop a second stable electrographic image on said non-insulating photoconductive layer.
 4. A method as defined in claim 3 wherein said original is formed from a photoconductive layer which is exposed to a radiation forming said conductivity pattern in said photoconductive layer.
 5. A method for producing electrographic images from an original provided with a conductivity pattern formed by areas on said original having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, comprising the steps of placing a layer of developer particles capable of receiving an electric charge against said conductivity pattern and against a non-insulating photoconductive layer so that said layer of developer particles is sandwiched between said conductivity pattern and said non-insulating photoconductive layer, said original and said non-insulating photoconductive layer being disposed between a first and a second electrode, generating across said original and said non-insulating photoconductive layer and between said electrodes a first electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said non-insulating photoconductive layer, simultaneously whereby an electrographic image is developed on said non-insulating photoconductive layer by a part of said particles, removing said electrographic image bearing non-insulating photoconductive layer from said first electric field, placing a copy material against the particles of said electrographic image, uniformly exposing said non-insulating photoconductive layer to radiation thereby inducing in said non-insulating photoconductive layer a high uniform conductivity and generating across said copy material and said non-insulating photoconductive layer a second electric field of sufficient strength to charge the particles of said electrographic image from said uniformly exposed non-insulating photoconductive layer thereby transferring said electrographic image onto said copy material.
 6. A method as defined in claim 5 wherein, simultaneously with the generation of said first electric field, said non-insulating photoconductive layer is uniformly exposed to radiation thereby inducing in this non-insulating photoconductive layer a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern.
 7. A method for producing electrographic images comprising the steps of placing a layer of developer particles capable of receiving an electric charge against a non-insulating photoconductive layer and against a conductive image carrier so that said layer of developer particles is sandwiched between said non-insulating photoconductive layer and said image carrier, exposing said non-insulating photoconductive layer to radiation thereby forming a conductivity pattern in said non-insulating photoconductive layer, said conductivity pattern being characterized by areas on said non-insulating photoconductive layer having differing electric conductivity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, said image carrier being formed from a material having a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern, said non-insulating photoconductive layer and said image carrier being disposed between a first and second electrodes, generating across said non-insulating photoconductive layer and said image carrier and between said electrodes a first electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said image carrier, simultaneously, whereby an electrographic image is developed on said non-insulating photoconductive layer by a part of said particles, removing said electrographic image bearing non-insulating photoconductive layer from said first electric field, placing a copy material against the particles of said electrographic image, uniformly exposing said non-insulating photoconductive layer to radiation thereby inducing a high uniform conductivity in said non-insulating photoconductive layer, and generating across said copy material and said non-insulating photoconductive layer a second electric field of sufficient strength to charge the particles of said electrographic image from said uniformly exposed non-insulating photoconductive layer thereby transferring said electrographic image onto said copy material.
 8. A method as defined in claim 7 wherein said non-insulating photoconductive layer is exposed to an image of visible radiation thereby forming said conductivity pattern in said non-insulating photoconductive layer during the generation of said first electric field, and said first electric field is generated to form on said non-insulating photoconductive layer a positive particles image to be transferred to said copy material.
 9. A method for producing electrographic images comprising the steps of placing a layer of developer particles capable of receiving an electric charge against a first and against a second non-insulating photoconductive layer so that said layer of developer particles is sandwiched between said first and said second non-insulating photoconductive layer, exposing said first non-insulating photoconductive layer to radiation thereby forming a conductivity pattern in said first non-insulating photoconductive layer, said conductivity pattern being characterized by areas on said first non-insulating photoconductive layer having differing electric conductIvity characteristics ranging from an area exhibiting maximum conductivity to an area exhibiting minimum conductivity, said first and second non-insulating photoconductive layer being disposed between a first and second electrodes, generating across said first and said second non-insulating photoconductive layer and between said electrodes a first electric field of sufficient strength to charge said layer of developer particles from said conductivity pattern and said second non-insulating photoconductive layer, simultaneously, whereby a first electrographic image is developed on said first non-insulating photoconductive layer by a part of said particles and a second electrographic image is developer on said second non-insulating photoconductive layer by the remaining particles, removing said first and second non-insulating photoconductive layers from said first electric field, placing a first copy material against the particles of said first electrographic image, uniformly exposing said first non-insulating photoconductive layer to radiation thereby inducing a high uniform conductivity in said first non-insulating photocontive layer, generating across said first copy material and said first non-insulating photoconductive layer a second electric field of sufficient strength to charge the particles of said first electrographic image from said uniformly exposed first non-insulating photoconductive layer thereby transferring said first electrographic image onto said first copy material, placing a second copy material against the particles of said second electrographic image, uniformly exposing said second non-insulating photoconductive layer to radiation thereby inducing a high uniform conductivity in said second non-insulating photoconductive layer, generating across said second copy material and said second non-insulating photoconductive layer a third electric field of sufficient strength to charge the particles of said second electrographic image from said uniformly exposed second non-insulating photoconductive layer thereby transferring said second electrographic image onto said second copy material.
 10. A method as defined in claim 9, wherein, simultaneously with the generation of said first electric field, said second non-insulating photoconductive layer is uniformly exposed to radiation thereby inducing in this second non-insulating photoconductive layer a uniform conductivity between the maximum and the minimum conductivities which are included in said conductivity pattern. 